Polyethylenes made from ethylene and a series of xcex1-olefins using a catalyst that copolymerizes ethylene and xcex1-olefins usually have excellent melt processing characteristics, especially in uses where high zero shear viscosity and low high shear viscosity are desirable. These polyethylenes described herein are particularly suitable for blow molding, extrusion or making extruded blown film.
Polyethylenes are important items of commerce, these being produced in larger volumes than any other polymer. Many different grades of this polymer type are produced, these differing grades varying in many properties, including cost. For an overview of polyethylenes, see B. Elvers, et al., Ed., Ullmann""s Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A21, VCH Verlagsgesellschaft, Weinheim, 1992, p. 488-518; and H. Mark et al., Ed., Encyclopedia of Polymer Science and Engineering, Vol. 6, John Wiley and Sons, New York, 1986, p. 383-489. Aside from cost the two major property areas of concern to most users (polymer processors) are final polymer physical properties (that is, does the polymer have physical properties suitable for the end use), and how difficult is it to form the polymer in the final article, often called processability. Oftentimes, the polymer user must compromise between better physical properties and better processability.
For example, the polyethylene with the highest tensile strength and highest use temperature (because of its relatively high melting point) is usually high density polyethylene (HDPE), which in its simplest form is the substantially linear polymer derived from ethylene. It is typically made by coordination polymerization of ethylene at moderate pressures (a few MPa) and temperatures (typically 50-150xc2x0 C.). However its melt processability is relatively poor.
At the other end of the polyethylene scale is low density polyethylene (LDPE) which is made at very high pressures (a hundred or more MPa) and high temperatures (approximately 200xc2x0 C.), necessitating the use of very expensive manufacturing plants and high operating costs to compress the ethylene for the polymerization. LDPE has relatively poor physical properties because it is highly branched, and these branches are believed to include both long chain branches (LCB, about 100 carbon atoms or more) and short chain branches (SCB,  less than  less than 100 carbon atoms). However the presence of the branching, especially it is believed the LCBs, render LDPE what is generally considered to be the best processing type of polyethylene.
There have been many attempts to combine the best properties of HDPE and LDPE. One of these resulted in linear low density polyethylene (LLDPE), a copolymer of ethylene and a lower xcex1-olefin, such as 1-butene, 1-hexene or 1-octene. Its processing is better than HDPE while not as good as LDPE, while its physical properties are better than LDPE but not as good as HDPE. Again a compromise (in a single polymer) is made between processability and physical properties.
For low cost, large volume polymers such as polyethylene, minimizing processing costs to form a shaped article is critical, since processing costs are a significant portion of the cost of the final shaped part. This means that processes with high throughput rates are particularly important, since they minimize cost. Among such processes are extrusion to produces so-called profile parts, blown film extrusion (in a sense a specialized form of extrusion), and blow molding which may be used to produce hollow parts such as bottles and other containers. The faster one can run these processes, the cheaper the produced parts will be. In the first part of melt forming in each of these processes, the polymer is extruded through a die under relatively high shear. The lower the viscosity of the polymer at high shear rate, the faster the polymer may be extruded at reasonable die pressures and without melt fracture. After exiting the die in each of these processes, and before the polymer solidifies, it is important that the polymer not deform (except when deformation is wanted, as when blowing the film, or blowing the hollow shape in blow molding) so as to hold the desired shape. For this part of the process one prefers to have a polymer with high melt viscosity at low shear. LDPE typically has just these properties, which is believed to be the reason it processes so well. Further it is believed in the art that these properties are the result of branching, especially LCB, in this polymer.
As mentioned above, LDPE is made in plants that are especially expensive to build and operate, and so LDPE tends to be more expensive than other grades of PE. A PE which could be made at lower pressures than LDPE is presently made, and which has the processability of LDPE, would therefore be advantageous. In addition if that polymer has better physical properties than LDPE, it would be even more advantageous.
U.S. Pat. No. 6,103,946 describes the oligomerization of ethylene with certain iron catalysts to form xcex1-olefins. WO99/50318 (corresponding to U.S. patent application Ser. No. 09/273,409, filed Mar. 22, 1999, now U.S. Pat. No. 6,214,761) describes the manufacture of a branched polythylene by reaction of ethylene with a selected iron catalyst and a selected polymerization catalyst capable of copolymerizing ethylene and xcex1-olefins. The above references are incorporated by reference herein for all purposes as if fully set forth.
Various reports of xe2x80x9csimultaneousxe2x80x9d oligomerization and polymerization of ethylene to form (in most cases) branched polyethylenes have appeared in the literature. See for instance WO90/15085, WO99/50318, U.S. Pat. No. 5,753,785, U.S. Pat. No. 5,856,610, U.S. Pat. No. 5,686,542, U.S. Pat. No. 5,137,994 and U.S. Pat. No. 5,071,927; C. Denger, et al, Makromol. Chem. Rapid Commun., vol. 12, p. 697-701 (1991); and E. A. Benham, et al., Polymer Engineering and Science, vol. 28, p. 1469-1472 (1988).
None of the above references recognizes that any of the polymers made have exceptional rheological properties.
This invention concerns an improved process for blow molding, extrusion or making extruded blown film of a polyethylene, by forming a molten polyethylene into a useful shape using this process, wherein the improvement comprises, using as said polyethylene a polymer made by copolymerizing ethylene and a series of olefins of the formula H2Cxe2x95x90CHR18, wherein R18 is n-alkyl containing an even number of carbon atoms, and provided that:
said series of olefins contain at least four different olefins; and
that in at least two different olefins of said series of olefins R18 contains 10 or more carbon atoms.
This invention also concerns an improved process for blow molding, extrusion or making extruded blown film of a polyethylene, by forming a molten polyethylene into a useful shape using this process, wherein the improvement comprises, using as said polyethylene which has one or both of a structural index, ST (as defined herein), of about 1.4 or more, and a processability index, PR (as defined herein) of about 40 or more, provided that if ST is less than about 1.4, said polymer has fewer than 20 methyl branches per 1000 methylene groups.
Also disclosed herein is an improved process for blow molding, extrusion or making extruded blown film of a polyethylene, by forming a molten polyethylene into a useful shape using this process, wherein the improvement comprises, using as said polyethylene a polyethylene which has at least 2 branches each of ethyl and n-hexyl or longer and at least one n-butyl per 1000 methylene groups, and has fewer than 20 methyl branches per 1000 methylene groups, and obeys the equation
[xcex7] less than 0.0007Mw0.66
wherein [xcex7] is the intrinsic viscosity in 1,2,4-trichlorbenzene at 150xc2x0 C. and Mw is the weight average molecular weight.